The more microplastic types pollute the soil, the stronger the growth suppression of invasive alien and native plants

Abstract The ecological consequences of microplastic pollution for plants remain largely unknown, and the few studies that tested the effects usually focused on a single type of microplastic and a single plant species. However, most plants will be exposed to multiple microplastic types simultaneously, and the effects may vary among species. To test the effects of microplastic diversity on plants, we grew single plants of eight invasive and eight native species in pots with substrate polluted with 0, 1, 3 and 6 types of microplastics. We found that the growth suppression by microplastic pollution became stronger with the number of microplastic types the plants were exposed to. This tended to be particularly the case for invasive species, as their biomass advantage over natives diminished with the number of microplastic types. The biomass responses coincided with a positive effect of the number of microplastic types on root allocation and thickness, which was also stronger for invasive than for native species. In addition, the results of hierarchical diversity–interaction models suggest that the negative impact of microplastic diversity on the total biomass of invasive plant species was influenced by both the identities of the microplastic and certain types of microplastic with strong pairwise interactions. In contrast, the effect on native species was determined solely by the microplastic identities. Synthesis . Our multispecies study thus shows for the first time that the negative effects of microplastic pollution on plant growth increase with the number of microplastic types. We also found tentative evidence that the negative impacts of microplastic diversity were more pronounced for invasive plants compared to native plants, and that this might be due to differences in the responses of root allocation and thickness.